ABSTRACT: Intensive study of the highly endangered western North Atlantic right whale Eubalaena glacialis over the past 25 yr has yielded evidence of reproductive dysfunction and compromised health, particularly in the late 1990s. Among the factors identified as potentially contributing to this phenomenon, exposure to marine biotoxins associated with harmful algal blooms has received little consideration. We assessed the occurrence of paralytic shellfish poisoning (PSP) toxins (saxitoxin [STX] analogues) in E. glacialis and in the co-occurring zooplankton assemblage dominated by Calanus finmarchicus, the primary food for this whale species in the North Atlantic. Samples of E. glacialis feces collected during August/September 2001 from at least 11 different whales in the Bay of Fundy, Canada, tested positive for PSP toxins using a receptor binding assay and were also quantified by high-performance liquid chromatography with fluorescence detection, indicating concentrations as high as 0.5 µg STX equivalents g1 of feces. Zooplankton samples collected in the Bay of Fundy during the same period contained similar levels of PSP toxins by weight using both methods. Additional data from the Bay of Fundy revealed the presence of PSP toxin-producing dinoflagellates, Alexandrium spp., immediately before and during the sampling period. Associated PSP toxin levels in shellfish from nearby Cheney Passage, New Brunswick, exceeded regulatory limits over the same time frame. These findings provide direct evidence for the occurrence of PSP toxins in E. glacialis and suggest that trophic transfer of marine algal toxins is a factor contributing to the failure of the endangered North Atlantic right whale population to recover.